1. Field of the Invention
The present invention relates to a radiation generating tube, and a radiation generating apparatus and a radiation imaging system including the radiation generating tube, which can be used for medical equipment, a nondestructive inspection apparatus, and the like, for example. This application claims the benefit of Japanese Patent Application No. 2013-123299, filed Jun. 12, 2013, which is incorporated by reference herein in its entirety.
2. Description of the Related Art
In general, a radiation generating apparatus includes a radiation generating tube as a radiation source. The radiation generating tube includes a vacuum container in which a cathode is mounted to one opening of an insulating tube and an anode is mounted to the other opening of the insulating tube. An electron emitting source is connected to the cathode, and the anode includes a target. The radiation generating tube irradiates the target with an electron emitted from the electron emitting source by applying a high voltage between the cathode and the anode, and thus generates a radiation such as an X-ray.
In order to keep an electron emitting property of the electron emitting source functioning, it is necessary to at least maintain or improve the joining reliability of elements when producing the vacuum container. For example, in U.S. Patent Application Publication No. 2007/0076849, there is disclosed a configuration in which a cathode of an insulating tube is joined to the insulating tube in both a longitudinal direction and a radial direction of the insulating tube.
When an electron beam emitted from the electron emitting source strikes the target, the target generates a radiation. In this case, most of the radiation changes to heat, and hence the target generates heat, which is transmitted to the anode holding the target to increase the temperature of the anode.
The radiation generating apparatus is filled with an insulating fluid serving as a refrigerant so as to cool a high-temperature portion of the radiation generating tube to equalize the temperature of the entire radiation generating tube. However, a rapid increase in temperature of the anode is not sufficiently handled by the temperature equalization by the insulating fluid in some cases. Specifically, in the case of rapid imaging in accordance with the movement of a circulatory organ or the like and the movement of a patient's body in a medical X-ray imaging apparatus, a short pulse and a large tube current are required so as to enhance a resolution, and a target increases in temperature rapidly in some cases. Also in the industrial field, there is a demand for a transmission inspection image having high resolution and for a high density packaging in an integrated device. For this purpose, it is requested to perform imaging with a focused electron beam entering a target, and hence the target increases in temperature rapidly in some cases.
The insulating tube formed of an insulator such as ceramics has a linear expansion coefficient lower than that of the anode formed of a conductor such as a metal. Therefore, in the case where the temperature of the anode and the insulating tube increases, thermal stress is caused due to the difference in linear expansion coefficient. Such thermal stress is concentrated on a joint portion having a low fracture toughness value between the insulating tube and the anode. Further, as described above, in the case where the anode increases in temperature rapidly, a temperature difference between the anode and the insulating tube is widened, and the thermal stress caused by the difference in linear expansion coefficient further increases, with the result that the joint portion between the insulating tube and the anode is subjected to damage such as cracks in some cases. Such damage to the insulating tube lowers the vacuum degree of the vacuum container, which is one cause for impairing the reliability of the radiation generating apparatus.
Also in the cathode, in the case where a hot cathode type electron emitting source is used, the electron emitting source generates heat by being driven, and the cathode increases in temperature rapidly. As a result, in the same way as in the anode, thermal stress is concentrated on the periphery of a joint portion between the insulating tube and the cathode, and the insulating tube is subjected to damage such as cracks in some cases.